Shock wave visualization for extreme ultraviolet plasma optimization

ABSTRACT

A method for monitoring a shock wave in an extreme ultraviolet light source includes irradiating a target droplet in the extreme ultraviolet light source apparatus of an extreme ultraviolet lithography tool with ionizing radiation to generate a plasma and to detect a shock wave generated by the plasma. One or more operating parameters of the extreme ultraviolet light source is adjusted based on the detected shock wave.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a Continuation application of U.S. application Ser.No. 16/655,116, filed Oct. 16, 2019, which claims priority to U.S.Provisional Application No. 62/752,289, filed on Oct. 29, 2018, entitled“Shock Wave Visualization for Extreme Ultraviolet Plasma Optimization,”the entire disclosure of each of which are incorporated herein byreference.

BACKGROUND

The wavelength of radiation used for lithography in semiconductormanufacturing has decreased from ultraviolet to deep ultraviolet (DUV)and, more recently to extreme ultraviolet (EUV). Further decreases incomponent size require further improvements in resolution of lithographywhich are achievable using extreme ultraviolet lithography (EUVL). EUVLemploys radiation having a wavelength of about 1-100 nm. One method forproducing EUV radiation is laser-produced plasma (LPP). In an LPP-basedEUV source, a high-power laser beam is focused on small droplet targetsof metal, such as tin, to form a highly ionized plasma that emits EUVradiation with a peak maximum emission at 13.5 nm.

The intensity of the EUV radiation produced by LPP depends on theeffectiveness with which the high-powered laser can produce the plasmafrom the droplet targets. Precise synchronization of the pulses of thehigh-powered laser with generation and movement of the droplet targetsis desired to improve the efficiency of an LPP-based EUV radiationsource. The laser-produced plasma may generate a shock wave in theLPP-based EUV source and the momentum carried by the shock wave may betransferred to the next droplet and cause droplet position deviationsuch that the next laser pulse may not efficiently hit the next dropletor may even miss the next droplet. A monitoring system to determine whena shock wave is generated and the extent of the shock wave and a controlmethod to minimize the effect of the shock wave is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 shows a schematic view of an EUV lithography system with a laserproduced plasma (LPP) EUV radiation source in accordance with someembodiments of the present disclosure.

FIG. 2 shows a schematic view of an EUV lithography exposure tool inaccordance with some embodiments of the present disclosure.

FIG. 3 shows a cross-sectional view of a reflective mask in accordancewith some embodiments of the present disclosure.

FIGS. 4A and 4B show cross-sectional views of the EUV radiation sourcein an operation situation in accordance with some embodiments of thepresent disclosure.

FIGS. 5A and 5B show schematic views of plasma and shock wave formationprocess through laser-metal interaction between a laser beam and a metaldroplet in accordance with some embodiments of the present disclosure.

FIG. 6 shows a schematic view of a collector mirror and related portionsof an EUV radiation source in accordance with some embodiments of thepresent disclosure.

FIG. 7A shows devices for illuminating and imaging tin droplets andshock waves in an EUV radiation source in accordance with someembodiments of the present disclosure.

FIG. 7B shows devices for illuminating and imaging tin droplets in anEUV radiation source in accordance with some embodiments of the presentdisclosure.

FIG. 8A shows devices for illuminating and imaging the shock waves inthe EUV radiation source in accordance with some embodiments of thepresent disclosure.

FIG. 8B is a detailed view of the slit control mechanism used in theapparatus of FIG. 8A in accordance with an embodiment of the presentdisclosure.

FIG. 9A shows an optical system for illuminating and imaging the shockwaves in the EUV radiation source in accordance with some embodiments ofthe present disclosure.

FIG. 9B shows an aperture of the optical system for imaging the shockwaves in the EUV radiation source in accordance with some embodiments ofthe present disclosure.

FIG. 10 shows a system for measuring a strength and direction of a shockwave, measuring a velocity and direction of droplets, and controlling anEUV radiation source, in accordance with some embodiments of the presentdisclosure.

FIG. 11 illustrates a flow diagram of an exemplary process for detectinga shock wave in a EUV radiation source, in accordance with someembodiments of the present disclosure.

FIGS. 12A and 12B illustrate an apparatus for controlling and monitoringan EUV lithography system, in accordance with some embodiments of thepresent disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly. In addition, the term“being made of” may mean either “comprising” or “consisting of.” In thepresent disclosure, a phrase “one of A, B and C” means “A, B and/or C”(A, B, C, A and B, A and C, B and C, or A, B and C), and does not meanone element from A, one element from B and one element from C, unlessotherwise described.

The present disclosure is generally related to extreme ultravioletlithography (EUVL) systems and methods. More particularly, it is relatedto apparatuses and methods for monitoring the metal droplets, e.g., tindroplets, that are travelling from a droplet generator to a zone ofexcitation where at the zone of excitation a excitation laser hits thedroplets, interacts with (heats) the tin droplet in the LPP chamber toionize the droplets to a plasma which emits the EUV radiation. In someembodiments, the interaction of the excitation laser with the tindroplet creates a shock wave. In some embodiments, the shock waveexpands and impacts the next tin droplet that is generated by thedroplet generation such that the direction of travel and/or speed of thenext droplet changes such that the next droplet does not pass throughthe zone of excitation at the time that the excitation laser is fired.

A droplet illumination module (DIM) is used to illuminate the inside ofthe EUV radiation source and a droplet detection module (DDM) is used tomeasure the parameters corresponding to the tin droplets. The DIMdirects non-ionizing light, e.g., a laser light, to the target dropletand the reflected and/or scattered light is detected by the DDM. Thelight from the DIM is “non-ionizing” and the light from the DIM is usedto illuminate the metal droplets inside the EUVL system such that adetector such as a camera can take an image of the tin droplets. Theembodiments of the present disclosure are directed to controllingdroplet illumination and detection for accurately measuring theparameters related to the metal droplets that include direction oftravel and speed of the droplets inside the EUVL system.

As noted, the interaction of the excitation laser with the tin dropletmay create a shock wave. A shock wave illumination module (SWIM) is usedto illuminate the zone of excitation inside of the EUV radiation sourceand a shock wave detection module (SWDM) is used to image the zone ofexcitation and to determine if a shock wave is generated and if theshock wave may impact the direction of travel of the droplets. In someembodiments, if it is determined that a shock wave is generated and itis determined that the shock wave impacts the direction of travel and/orspeed of the next droplet, a controller commands the droplet generatorto delay the generation of the next droplet until the shock wave clearsa path of the next droplet. Also, the controller commands the lasergenerator to delay the next pulse of the excitation laser.

FIG. 1 shows a schematic view of an EUV lithography system with a laserproduced plasma (LPP) EUV radiation source in accordance with someembodiments of the present disclosure. The EUV lithography systemincludes an EUV radiation source 100 (an EUV light source) to generateEUV radiation, an exposure device 200, such as a scanner, and anexcitation laser source 300. As shown in FIG. 1, in some embodiments,the EUV radiation source 100 and the exposure device 200 are installedon a main floor MF of a clean room, while the excitation laser source300 is installed in a base floor BF located under the main floor. Eachof the EUV radiation source 100 and the exposure device 200 are placedover pedestal plates PP1 and PP2 via dampers DMP1 and DMP2,respectively. The EUV radiation source 100 and the exposure device 200are coupled to each other by a coupling mechanism, which may include afocusing unit.

The lithography system is an EUV lithography system designed to expose aresist layer by EUV light (also interchangeably referred to herein asEUV radiation). The resist layer is a material sensitive to the EUVlight. The EUV lithography system employs the EUV radiation source 100to generate EUV light, such as EUV light having a wavelength rangingbetween about 1 nm and about 100 nm. In one particular example, the EUVradiation source 100 generates an EUV light with a wavelength centeredat about 13.5 nm. In the present embodiment, the EUV radiation source100 utilizes a mechanism of laser-produced plasma (LPP) to generate theEUV radiation.

The exposure device 200 includes various reflective optical components,such as convex/concave/flat mirrors, a mask holding mechanism includinga mask stage, and wafer holding mechanism. The EUV radiation generatedby the EUV radiation source 100 is guided by the reflective opticalcomponents onto a mask secured on the mask stage. In some embodiments,the mask stage includes an electrostatic chuck (e-chuck) to secure themask. Because gas molecules absorb EUV light, the lithography system forthe EUV lithography patterning is maintained in a vacuum or a-lowpressure environment to avoid EUV intensity loss. The exposure device200 is described in more details with respect to FIG. 2.

In the present disclosure, the terms mask, photomask, and reticle areused interchangeably. In some embodiments, the mask is a reflectivemask. In some embodiments, the mask includes a substrate with a suitablematerial, such as a low thermal expansion material or fused quartz. Invarious examples, the material includes TiO₂ doped SiO₂, or othersuitable materials with low thermal expansion. The mask includesmultiple reflective layers (ML) deposited on the substrate. The MLincludes a plurality of film pairs, such as molybdenum-silicon (Mo/Si)film pairs (e.g., a layer of molybdenum above or below a layer ofsilicon in each film pair). Alternatively, the ML may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configurable to highly reflect the EUV light. The mask mayfurther include a capping layer, such as ruthenium (Ru), disposed on theML for protection. The mask further includes an absorption layer, suchas a tantalum boron nitride (TaBN) layer, deposited over the ML. Theabsorption layer is patterned to define a layer of an integrated circuit(IC). Alternatively, another reflective layer may be deposited over theML and is patterned to define a layer of an integrated circuit, therebyforming an EUV phase shift mask.

The exposure device 200 includes a projection optics module for imagingthe pattern of the mask on to a semiconductor substrate with a resistcoated thereon secured on a substrate stage of the exposure device 200.The projection optics module generally includes reflective optics. TheEUV radiation (EUV light) directed from the mask, carrying the image ofthe pattern defined on the mask, is collected by the projection opticsmodule, thereby forming an image on the resist.

In various embodiments of the present disclosure, the semiconductorsubstrate is a semiconductor wafer, such as a silicon wafer or othertype of wafer to be patterned. The semiconductor substrate is coatedwith a resist layer sensitive to the EUV light in presently disclosedembodiments. Various components including those described above areintegrated together and are operable to perform lithography exposingprocesses. The lithography system may further include other modules orbe integrated with (or be coupled with) other modules.

As shown in FIG. 1, the EUV radiation source 100 includes a dropletgenerator 115 and a LPP collector mirror 110, enclosed by a chamber 105.A droplet DP that does not interact goes to a droplet catcher 85. Thedroplet generator 115 generates a plurality of target droplets DP, whichare supplied into the chamber 105 through a nozzle 117. In someembodiments, the target droplets DP are tin (Sn), lithium (Li), or analloy of Sn and Li. In some embodiments, the target droplets DP eachhave a diameter in a range from about 10 microns (μm) to about 100 μm.For example, in an embodiment, the target droplets DP are tin droplets,each having a diameter of about 10 μm, about 25 μm, about 50 μm, or anydiameter between these values. In some embodiments, the target dropletsDP are supplied through the nozzle 117 at a rate in a range from about50 droplets per second (i.e., an ejection-frequency of about 50 Hz) toabout 50,000 droplets per second (i.e., an ejection-frequency of about50 kHz). For example, in an embodiment, target droplets DP are suppliedat an ejection-frequency of about 50 Hz, about 100 Hz, about 500 Hz,about 1 kHz, about 10 kHz, about 25 kHz, about 50 kHz, or anyejection-frequency between these frequencies. The target droplets DP areejected through the nozzle 117 and into a zone of excitation ZE (e.g., atarget droplet location) at a speed in a range from about 10 meters persecond (m/s) to about 100 m/s in various embodiments. For example, in anembodiment, the target droplets DP have a speed of about 10 m/s, about25 m/s, about 50 m/s, about 75 m/s, about 100 m/s, or at any speedbetween these speeds.

The excitation laser beam LR2 generated by the excitation laser source300 is a pulsed beam. The laser pulses of laser beam LR2 are generatedby the excitation laser source 300. The excitation laser source 300 mayinclude a laser generator 310, laser guide optics 320 and a focusingapparatus 330. In some embodiments, the laser generator 310 includes acarbon dioxide (CO₂) or a neodymium-doped yttrium aluminum garnet(Nd:YAG) laser source with a wavelength in the infrared region of theelectromagnetic spectrum. For example, the laser source 310 has awavelength of 9.4 μm or 10.6 μm, in an embodiment. The laser light beamLRO generated by the excitation laser source 300 is guided by the laserguide optics 320 and focused, by the focusing apparatus 330, into theexcitation laser beam LR2 that is introduced into the EUV radiationsource 100. In some embodiments, in addition to CO₂ and Nd:YAG lasers,the laser beam LR2 is generated by a gas laser including an excimer gasdischarge laser, helium-neon laser, nitrogen laser, transversely excitedatmospheric (TEA) laser, argon ion laser, copper vapor laser, KrF laseror ArF laser; or a solid state laser including Nd:glass laser,ytterbium-doped glasses or ceramics laser, or ruby laser. In someembodiments, a non-ionizing laser beam LR1 is also generated by theexcitation laser source 300 and the laser beam LR1 is also focused bythe focusing apparatus 330.

In some embodiments, the excitation laser beam LR2 includes a pre-heatlaser pulse and a main laser pulse. In such embodiments, the pre-heatlaser pulse (interchangeably referred to herein as the “pre-pulse) isused to heat (or pre-heat) a given target droplet to create alow-density target plume with multiple smaller droplets, which issubsequently heated (or reheated) by a pulse from the main laser (mainpulse), generating increased emission of EUV light compared to when thepre-heat laser pulse is not used.

In various embodiments, the pre-heat laser pulses have a spot size about100 μm or less, and the main laser pulses have a spot size in a range ofabout 150 μm to about 300 μm. In some embodiments, the pre-heat laserand the main laser pulses have a pulse-duration in the range from about10 ns to about 50 ns, and a pulse-frequency in the range from about 1kHz to about 100 kHz. In various embodiments, the pre-heat laser and themain laser have an average power in the range from about 1 kilowatt (kW)to about 50 kW. The pulse-frequency of the excitation laser beam LR2 ismatched with the ejection-frequency of the target droplets DP in anembodiment.

The laser beam LR2 is directed through windows (or lenses) into the zoneof excitation ZE. The windows adopt a suitable material substantiallytransparent to the laser beams. The generation of the laser pulses issynchronized with the ejection of the target droplets DP through thenozzle 117. As the target droplets move through the excitation zone, thepre-pulses heat the target droplets and transform them into low-densitytarget plumes. A delay between the pre-pulse and the main pulse iscontrolled to allow the target plume to form and to expand to an optimalsize and geometry. In various embodiments, the pre-pulse and the mainpulse have the same pulse-duration and peak power. When the main pulseheats the target plume, a high-temperature plasma is generated. Theplasma emits EUV radiation, which is collected by the collector mirror110. The collector mirror 110, an EUV collector mirror, further reflectsand focuses the EUV radiation for the lithography exposing processesperformed through the exposure device 200.

One method of synchronizing the generation of a pulse (either or both ofthe pre-pulse and the main pulse) from the excitation laser with thearrival of the target droplet in the zone of excitation is to detect thepassage of a target droplet at given position and use it as a signal fortriggering an excitation pulse (or pre-pulse). In this method, if, forexample, the time of passage of the target droplet is denoted by to, thetime at which EUV radiation is generated (and detected) is denoted byt_(rad), and the distance between the position at which the passage ofthe target droplet is detected and a center of the zone of excitation isd, the speed of the target droplet, v_(dp), is calculated as

v _(dp) =d/(t _(rad)-t _(o))   Equation (1).

Because the droplet generator 115 is expected to reproducibly supplydroplets at a fixed speed, once v_(dp) is calculated, the excitationpulse is triggered with a time delay of d/v_(dp) after a target dropletis detected to have passed the given position to ensure that theexcitation pulse arrives at the same time as the target droplet reachesthe center of the zone of excitation. In some embodiments, because thepassage of the target droplet is used to trigger the pre-pulse, the mainpulse is triggered following a fixed delay after the pre-pulse. In someembodiments, the value of target droplet speed v_(dp) is periodicallyrecalculated by periodically measuring t_(rad), if needed, and thegeneration of pulses with the arrival of the target droplets isresynchronized.

In an EUV radiation source 100, the plasma caused by the laserapplication creates debris, such as ions, gases and atoms of thedroplet, as well as the desired EUV radiation. It is necessary toprevent the accumulation of material, e.g., debris, on the collectormirror 110 and also to prevent debris exiting the chamber 105 andentering the exposure device 200.

As shown in FIG. 1, a buffer gas is supplied from a first buffer gassupply 130 through the aperture in collector mirror 110 by which thepulse laser is delivered to the tin droplets. In some embodiments, thebuffer gas is H₂, He, Ar, N or another inert gas. In certainembodiments, H₂ is used as H radicals that are generated by ionizationof the buffer gas and can be used for cleaning purposes. The buffer gascan also be provided through one or more second buffer gas supplies 135toward the collector mirror 110 and/or around the edges of the collectormirror 110. Further, the chamber 105 includes one or more gas outlets140 so that the buffer gas is exhausted outside the chamber 105.Hydrogen gas has low absorption to the EUV radiation. Hydrogen gasreaching to the coating surface of the collector mirror 110 reactschemically with a metal of the droplet forming a hydride, e.g., metalhydride. When tin (Sn) is used as the droplet, stannane (SnH₄), which isa gaseous byproduct of the EUV generation process, is formed. Thegaseous SnH₄ is then pumped out through the gas outlet 140. However, itis difficult to exhaust all gaseous SnH₄ from the chamber and to preventthe SnH₄ from entering the exposure device 200. Therefore, monitoringand/or control of the debris in the EUV radiation source 100 isbeneficial to the performance of the EUVL system.

FIG. 2 shows a schematic view of an EUV lithography exposure tool inaccordance with some embodiments of the present disclosure. The EUVLexposure tool of FIG. 2 includes the exposure device 200 that shows theexposure of photoresist coated substrate, a target semiconductorsubstrate 210, with a patterned beam of EUV light. The exposure device200 is an integrated circuit lithography tool such as a stepper,scanner, step and scan system, direct write system, device using acontact and/or proximity mask, etc., provided with one or more optics205 a, 205 b, for example, to illuminate a patterning optic, such as areticle, e.g., a reflective mask 205 c, with a beam of EUV light, toproduce a patterned beam, and one or more reduction projection optics205 d, 205 e, for projecting the patterned beam onto the targetsemiconductor substrate 210. A mechanical assembly (not shown) may beprovided for generating a controlled relative movement between thetarget semiconductor substrate 210 and patterning optic, e.g., areflective mask 205 c. As further shown, the EUVL exposure tool of FIG.2, further includes the EUV radiation source 100 including a plasmaplume 23 at the zone of excitation ZE emitting EUV light in the chamber105 that is collected and reflected by a collector mirror 110 into theexposure device 200 to irradiate the target semiconductor substrate 210.

FIG. 3 shows a cross-sectional view of a reflective mask in accordancewith some embodiments of the present disclosure. The terms mask,photomask, and reticle may be used interchangeably. In some embodiments,the mask is a reflective mask as shown by mask 205 c in FIG. 2. Thereflective mask 205 c of FIG. 2 is shown in FIG. 3 and includes asubstrate 30, multiple reflective multiple layers (ML) 35 that aredeposited on the substrate 30, a conductive backside coating 60, acapping layer 40, and an absorption layer 45. In some embodiments, thematerial of the substrate 30 includes TiO₂ doped SiO₂, or other suitablematerials with low thermal expansion. In some embodiments, the ML 35includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si)film pairs (e.g., a layer of molybdenum 39 above or below a layer ofsilicon 37 in each film pair). Alternatively, the ML 35 may includemolybdenum-beryllium (Mo/Be) film pairs, or other suitable materialsthat are configured to highly reflect the EUV light. The capping layer40 may include ruthenium (Ru) and may be disposed on the ML 35 forprotection. In some embodiments, the absorption layer 45 that includes atantalum boron nitride (TaBN) layer is deposited over the ML 35 and thecapping layer 40. In some embodiments, the absorption layer 45 ispatterned with patterns 55 to define a layer of an integrated circuit(IC). In some embodiments, the reflective mask 205 c includes aconductive backside coating 60. In some embodiments, another reflectivelayer may be deposited over the ML 35 and to be patterned to define alayer of an integrated circuit, thereby forming an EUV phase shiftreticle.

FIGS. 4A and 4B show cross-sectional views of the EUV radiation sourcein an operation situation in accordance with some embodiments of thepresent disclosure. In FIG. 4A, the EUV radiation source 100 includesthe focusing apparatus 330, the collector mirror 110, the dropletgenerator 115, an aperture 50, and a drain such as the droplet catcher85, e.g., a tin catcher, for receiving the unreacted tin droplets, e.g.,the debris droplet 25. In some embodiments, the aperture 50 is used forthe laser beam LR2 of the focusing apparatus 330 and gas flow 132 toenter into the EUV radiation source 100. The walls 146 are used tocreate a cone shape such that the EUV radiation along the arrows 29inside the cone shape exit through the opening 148 and any otherradiation that is not along the arrows 29 do not exit the cone shape andthus do not exit the EUV radiation source 100. In some embodiments, atleast a portion of the gas flow 132 exits through openings 142 in thewall of the cone shape. The gas flow that exits through the openings 142may flow in the EUV radiation source 100 and exit the EUV radiationsource 100 through the gas outlet 140. The collector mirror 110 is madeof a multi-layered mirror including Mo/Si, La/B, La/B₄C, Ru/B₄C, Mo/B₄C,Al₂O₃/B₄C, W/C, Cr/C, and Cr/Sc with a capping layer including SiO₂, Ru,TiO₂, and ZrO₂, in some embodiments. The diameter of the collectormirror 110 can be about 330 mm to about 750 mm depending on the chambersize of the EUV radiation source 100. The cross-sectional shape of thecollector mirror 110 can be elliptical or parabolic, in someembodiments.

In some embodiments, the interaction between the laser beam LR2 and adroplet DP that creates the plasma plume 23 that includes active andhighly charged particles or ions such as tin (Sn) ions, also creates ashock wave. The shock wave that is described with respect to FIGS. 5Aand 5B, may push the next droplets DP away and cause a spatialpositional error/tolerance between the tin droplet DP and the focusposition of the laser beam LR2 at the zone of excitation ZE. Also, insome embodiments, an error exists in synchronization between the pulsefrequency of the laser beam LR2 and the speed of the ejected tin dropletDP such that when the pulsed laser that is directed to the zone ofexcitation ZE fires, it misses some droplets and the droplets that havenot reached or have already passed the zone of excitation ZE, and thus,become debris droplets 25. The debris droplets 25, may deposit on thelower-half portion of the reflective surface of the collector mirror110. The deposited debris on the collector mirror 110 may deterioratethe reflective property of the collector mirror 110, thereby loweringthe power of EUV radiation source 100 for EUV photolithography of thetarget semiconductor substrate 210 of FIG. 2, and lowering the quality(such as critical dimension CD and line edge roughness LER) of patternsformed on the photo-sensitive coating (not shown) on the targetsemiconductor substrate 210.

In FIG. 4B, the EUV radiation source 100 includes the same components ofthe EUV radiation source 100 of FIG. 4A with at least a difference thatfocusing apparatus 330 focuses two laser beams. The focusing apparatus330 focuses the laser beam LR2 at the zone of excitation ZE and alsofocuses a non-ionizing laser beam LR1 at a location slightly before thezone of excitation ZE. Thus, the aperture 50 allows the ionizing laserbeam LR2 and the non-ionizing laser beam LR1 to enter the EUV radiationsource 100. In some embodiments, the non-ionizing laser beam LR1pre-heats the tin droplet DP and creates a pancake-shaped tin 27 fromthe droplet DP such that the pancake-shaped tin 27 better interacts withthe laser beam LR2 at the zone of excitation and a larger plasma plume23 and a stronger shock wave is created compared the plasma plume 23 andshock wave that is created in FIG. 4A. In some embodiments, the shockwave causes the laser beams LR1 and LR2 to miss the next droplet DPthereby generating debris droplets 25. In some embodiments, the laserbeam LR1 focuses on the next droplet DP and creates the pancake-shapedtin 27, however, the shock wave causes the laser beam LR2 to miss thepancake-shaped tin 27 and the missed pancake-shaped tin 27 becomes adebris droplet 25.

FIGS. 5A and 5B show schematic views of plasma and shock wave formationprocesses through laser-metal interaction between a laser beam and ametal droplet in accordance with some embodiments of the presentdisclosure. In FIG. 5A, the ejected metal droplet, e.g., the tin dropletDP, ejected from the droplet generator 115, reaches the zone ofexcitation ZE where it interacts with the laser beam LR2 to form aplasma plume 23 and a shock wave 28. The zone of excitation ZE is at afocus of the high-power and high-pulse-repetition-rate pulsed laser beamLR2. The laser beam LR2 interacts with the ejected tin droplet DP at thezone of excitation ZE in a space between collector mirror 110 and thewalls 146 of EUV radiation source 100 to form the plasma plume 23 whichemits EUV light rays 24 in all directions. During this laser-metalinteraction, a tin droplet DP could be missed by or not interactsufficiently with the laser beam LR2, thereby passing to a positionbelow the zone of excitation ZE in FIG. 5A, forming debris droplet 25.Also, some tin leftover from the plasma formation process can becomedebris 26. The debris droplet 25 and debris 26 can accumulate on thesurface of the EUV collector mirror, e.g., collector mirror 110 of FIG.1, deteriorating the reflective quality of the EUV collector mirror 110.

In FIG. 5B, the ejected metal droplet, e.g., the ejected tin droplet DP,after being heated by the pre-heat laser beam LR1 and becoming thepancake-shaped tin 27 reaches the zone of excitation ZE where thepancake-shaped tin 27 interacts with the laser beam LR2 to form theplasma plume 23 and the shock wave 28. The zone of excitation ZE is at afocus of the high-power and high-pulse-repetition-rate pulsed laser beamLR2. The laser beam LR2 interacts with the pancake-shaped tin 27 at thezone of excitation ZE in a space of the chamber of the EUVL system toform the plasma plume 23 which emits EUV light rays 24 in alldirections. During this laser-metal interaction, a pancake-shaped tin 27could be missed by or not interact sufficiently with the laser beam LR2,thereby passing to a position below the zone of excitation ZE in FIG.5B, forming debris droplet 25. Also, some tin leftover from the plasmaformation process can become debris 26. In some embodiments as shown inFIG. 5B, preheating the droplet DP to create the pancake-shaped tin 27causes a stronger interaction of the laser beam LR2 and thus creates abigger plasma plume 23, a stronger shock wave 28, and also a smallerdebris 26. As shown in FIGS. 5A and 5B the droplets DP travel in adirection 22 from droplet generator 115 to the zone of excitation ZE. Insome embodiments, the shock wave 28 creates an impact on the nextdroplet DP and moves the next droplet DP away from the direction 22 suchthat the next droplet DP does not pass through the zone of excitationZE.

FIG. 6 shows a schematic view of a collector mirror and related portionsof an EUV radiation source in accordance with some embodiments of thepresent disclosure. FIG. 6 shows a schematic view of the EUV radiationsource 100 that includes a debris collection mechanism 150, thecollector mirror 110 including the aperture 50, the droplet generator115, and the droplet catcher 85. The circled area 152 in FIG. 6 showsdrip holes 158. In some embodiments, the excess tin and molten debristhat pass through the debris collection mechanism 150 are collected viadrip holes 158. In some embodiments, the debris collection mechanism 150is consistent with or includes the walls 146 of FIGS. 4A and 4B.

FIG. 7A shows devices for illuminating and imaging tin droplets andshock waves in an EUV radiation source in accordance with someembodiments of the present disclosure. A plan view 700 of FIG. 7A is acut of the EUV radiation source 100 of FIG. 1. The plan view 700includes a DIM 710A, a DDM 720A, the collector mirror 110, and the tindroplets DP moving from the droplet generator 115 to the zone ofexcitation ZE. The DIM 710A provides a light beam 740 to illuminate thedroplets DP at the zone of excitation ZE and in a path between thedroplet generator 115 and the zone of excitation ZE. In someembodiments, the DIM 710A includes one or more light sources, includingone or more laser sources for illuminating the zone of excitation ZE andthe path between the droplet generator 115 and zone of excitation ZE. Insome embodiments, the DIM 710A includes collimating optics including oneor more lenses for illuminating, e.g., uniformly illuminating, the zoneof excitation ZE and the path between the droplet generator 115 and zoneof excitation ZE. In some embodiments, the DDM 720A includes one or moreimage sensors, including a camera, e.g., a digital camera. In someembodiments, the DDM 720A includes condensing optics and/or imagingoptics including one or more lenses for capturing the light reflectedfrom the droplets DP and to generate one or more images of the dropletsDP.

The plan view 700 also includes a shock wave illumination module (SWIM)710D and a shock wave detection module (SWDM) 720D. In some embodiments,the SWIM 710D is used for illuminating a shock wave, e.g., shock wave 28of FIG. 5A or 5B, that is generated around the zone of excitation ZEwhen the laser beam LR2 hits the droplet DP at the zone of excitationZE. In some embodiments, the SWIM 710D includes collimating opticsincluding one or more lenses for illuminating, e.g., uniformlyilluminating, at and around the zone of excitation ZE. In someembodiments, the SWDM 720D includes one or more image sensors (imagedetectors), including a camera, e.g., a digital camera. In someembodiments, the SWDM 720D includes condensing optics and/or imagingoptics including one or more lenses for capturing the light that passesthrough the shock wave 28 and to generate one or more images of theshock wave 28. Also, as shown in FIG. 7A, in addition to the SWDM 720D,at least one of the devices 720B or 720C is also installed around theEUV radiation source 100 such that the cameras of the devices 720B or720C may take images of the shock wave 28 from multiple viewpointsinside the EUV radiation source 100 to determine a plane of the shockwave 28. Imaging the shock wave 28 is described in more detail withrespect to FIGS. 8A, 8B, 9A, and 9B.

In some embodiments, a plurality of DIMs is installed around the EUVradiation source 100. As shown in FIG. 7A, in addition to the DIM 710A,devices 710B and 710C are also installed around the EUV radiation source100 such that the light sources of devices 710B and 710C illuminatedifferent views of the zone of excitation ZE and the path between thedroplet generator 115 and the zone of excitation ZE. Also, as shown inFIG. 7A, in addition to the DDM 720A, at least one of the devices 720Bor 720C is also installed around the EUV radiation source 100 such thatthe cameras of the devices 720B or 720C may take images of the dropletsDP from multiple viewpoints inside the EUV radiation source 100.

In some embodiments, the camera of the DDM 720A takes at least twoimages of the zone of excitation ZE and/or the path between the dropletgenerator 115 and zone of excitation ZE. Therefore, the two or moreimages taken by the camera of the DDM 720A may show different locationsof the droplets DP after being released from the droplet generator 115and before reaching the zone of excitation ZE. In some embodiments, thetwo images are taken successively, with a slight time difference betweenthem, and thus the images show how the droplets DP move from one imageto the next image in the path between the droplet generator 115 and zoneof excitation ZE. In some embodiments, a velocity and a location of thedroplets DP is determined based on the successive images.

FIG. 7B shows devices for illuminating and imaging tin droplets in anEUV radiation source in accordance with some embodiments of the presentdisclosure. The device 790 shows the DIM 710, which is consistent withDIM 710A of FIG. 7A and the DDM 720, which is consistent with the DDM720A of FIG. 7A. A light beam 740 of the DIM 710 illuminates the tindroplets DP in the EUV radiation source 100. The device 790 furthercaptures images, via the DDM 720, of the tin droplets DP in the EUVradiation source 100.

In some embodiments, a light source of the DIM 710 is used forilluminating, by light beam 740, the zone of excitation ZE and the pathbetween the droplet generator 115 and the zone of excitation ZE. Thedroplet DP ejected by from the nozzle 117 of the droplet generator 115moves in a direction 712 between the droplet generator 115 and the zoneof excitation ZE. The reflected or scattered light 722 from the one ormore droplets DP is captured by an image sensor, e.g., a camera, in theDDM 720. In some embodiments and consistent with FIG. 7A, one or moreother DIMs (not shown in FIG. 7B), having corresponding light sources,are included in the device 790 and the other DIMs are used to illuminateother parts and/or other views of the EUV radiation source 100. Also,consistent with FIG. 7A, one or more other DDMs (not shown) havingcorresponding image sensors, e.g., cameras, are included in the device790 and the other cameras are used for capturing the reflected orscattered light from other views of the one or more droplets DP. The useof additional light beams 740 of the light sources of the other DIMs andusing the cameras of the other DDMs allows images to be captured frommultiple viewpoints inside the EUV radiation source 100. As noted above,the camera of DDM 720 and the cameras of the other DDMs, take two ormore consecutive images, with a slight time difference betweenconsecutive images. Thus, in some embodiments, a controller 750 of thedevice 790 is used for determining a location and velocity of thedroplets DP. The velocity is determined by analyzing the capturedconsecutive images at one or more viewpoints and determining a distancethe same droplet DP has travelled between the consecutive images.

In some embodiments, when the laser beam LR2, the excitation laser beam,hits the target droplet DP within the zone of excitation ZE, the plasmaplume 23 forms because of ionization of the target droplet DP thatcauses the target droplet DP to expand rapidly into a plasma volume. Thevolume of the plasma plume 23 depends on the size of the target dropletDP and the energy provided by the laser beam LR2. In some embodiments,the plasma expands several hundred microns from the zone of excitationZE and creates the shock wave 28. As used herein, the term “expansionvolume” refers to a volume to which plasma plume 23 expands after thetarget droplets are heated with the excitation laser beam LR2.

In some embodiments, the DIM 710 includes a continuous wave laser. Inother embodiments, the DIM 710 includes a pulsed laser. The wavelengthof the laser of the DIM 710 is not particularly limited. In someembodiments, the laser of the DIM 710 has a wavelength in the visibleregion of electromagnetic spectrum. In some embodiments, the DIM 710 hasa wavelength of about 1070 nm. In some embodiments, the laser of the DIM710 has an average power in the range from about 1 W to about 50 W. Forexample, in some embodiments, the laser of the DIM 710 has an averagepower of about 1 W, about 5 W, about 10 W, about 25 W, about 40 W, about50 W, or any average power between these values. In some embodiments,the DIM 710 generates a beam having a uniform illumination profile. Forexample, in some embodiments, the DIM 710 creates a fan-shaped lightcurtain or a thin plane of light having substantially the same intensityacross its profile that illuminates the path between the dropletgenerator 115 and the zone of excitation ZE. In some embodiments, theDIM 710 creates a light beam with a substantially uniform intensityacross a volume that illuminates the path between the droplet generator115 and the zone of excitation ZE.

As the target droplet DP passes through the beam generated by the DIM710, the target droplet DP reflects and/or scatters the photons in thebeam. In an embodiment, the target droplet DP produces a substantiallyGaussian intensity profile of scattered photons. The photons scatteredby the target droplet DP are detected by the DDM 720. In someembodiments, the peak of the intensity profile detected by the DDM 720corresponds to the center of the target droplet DP. In some embodiments,the DDM 720 includes a photodiode and generates two or more consecutiveelectrical signals upon detecting the photons reflected and/or scatteredby the target droplet DP. In some embodiments, the DDM 720 includes acamera and generates two or more consecutive images of the photonsreflected and/or scattered by the target droplet DP.

In an embodiment, a synchronizer 730 synchronizes the illumination lightbeam 740 generated by the DIM 710 with the recording of the illuminationlight reflected from or scattered by the particles to the DDM 720. Insome embodiments, the controller 750 controls and synchronizes thesynchronizer 730, the triggering of the DIM 710 and the DDM 720, and thereleasing of tin droplets DP by the droplet generator 115. In addition,the controller 750 provides a trigger signal to the excitation lasersource 300 of FIG. 1 that generates the laser beam LR2 such that a laserpulse generating the laser beam LR2 is synchronized with the releasingof tin droplets DP, the DIM 710, and the DDM 720. In some embodiments,the controller 750 controls the DIM 710 and DDM 720 through thesynchronizer 730. In some embodiments, the synchronizer 730 does notexist and the controller 750 directly controls and synchronizes the DIM710, the DDM 720, the excitation laser source 300, and the dropletgenerator 115. In some embodiments, a compression controller device 732is attached to the nozzle 117 of the droplet generator 115. In someembodiments, the compression controller device 732 is made ofpiezoelectric material, such as a PZT, and by exerting a surface tensionat one end of the nozzle 117 determines when a droplet DP is released.Thus, the controller 750 may be coupled to the compression controllerdevice 732 and control when a droplet DP is released and also bymodifying a voltage of the compression controller device 732 may changea speed of the droplet DP when released.

In some embodiments, the digital camera of DDM 720 uses CCD or CMOSimage sensors and can capture two frames at high speed with a fewhundred nano-seconds difference between the frames.

In some embodiments, lasers are used as the light source of DIM 710 dueto their ability to produce high-power light beams with short pulsedurations. This yields short exposure times for each frame. In someembodiments, Nd:YAG lasers are used. The Nd:YAG lasers emit primarily atabout the 1064 nm wavelength and its harmonics (532, 266, etc.). Forsafety reasons, the laser emission is typically bandpass filtered toisolate the 532 nm harmonics (this is green light, the only harmonicable to be seen by the naked eye).

In some embodiments, the DIM 710 include optics for illuminating thepath between the droplet generator 115 and the zone of excitation ZE.The optics include one or more spherical lenses and/or a cylindricallens combination in some embodiments. The cylindrical lens expands thelaser into a plane while the spherical lens compresses the plane into athin sheet. A thickness of the plane is on the order of the wavelengthof the laser light and occurs at a finite distance from the optics setup(the focal point of the spherical lens), in some embodiments. In someembodiments, the lens for the camera of DDM 720 is arranged to properlyfocus on and visualize the droplets in the path between the dropletgenerator 115 and the zone of excitation ZE.

In some embodiments, the synchronizer 730 acts as an external triggerfor both the DDM 720 and the DIM 710. The controller 750 is arranged tocontrol the synchronizer 730, DIM 710, and DDM 720. In some embodiments,the synchronizer 730 sets the timing of each frame of the DDM 720sequence in conjunction with the firing of the illumination light sourceof DIM 710 to within 1 ns precision. Thus, the time between each pulseof the laser and the placement of the laser shot in reference to thecamera's timing can be accurately controlled to determine the velocityof the droplets. Stand-alone electronic synchronizers, called digitaldelay generators, offer variable resolution timing from as low as 250 psto as high as several milliseconds that can control the light source ofDIM 710 and the detector (e.g., camera) of the DDM 720 and may providemultiple camera exposures.

FIG. 8A shows devices for illuminating and imaging the shock waves inthe EUV radiation source in accordance with some embodiments of thepresent disclosure and FIG. 8B is a detailed view of the slit controlmechanism used in the apparatus of FIG. 8A in accordance with anembodiment of the present disclosure.

In some embodiments, the device of FIG. 8A includes a SWIM 810, a SWDM820, a controller 850 and a processor 800. In some embodiments, the SWIM810 includes a radiation source 855, a tilt control mechanism 853, aslit control mechanism 857, and an illumination system 805. The tiltcontrol mechanism 853 (also referred to herein as “auto tilt”) controlsthe tilt of the radiation source 855, which is consistent with the EUVradiation source 100 of FIG. 1. In some embodiments, the auto tilt 853is a stepper motor coupled to the radiation source 855 (e.g., a lasersource) of the SWIM 810 and moves the radiation source 855 to change theangle of incidence at which light beam L (e.g., radiation) is incidentat the plasma plume 23 and the shock wave 28 (and in effect changing theamount of light T passing through the plasma plume 23 and the shock wave28 and into the SWDM 820). In some embodiments, the auto tilt 853includes a piezoelectric actuator. In some embodiments, the illuminationsystem 805 receives a light beam L0, e.g., a laser beam, from radiationsource 855 and transforms the light beam L0 into light beam L, which isa collimated beam of light as shown in FIG. 9A.

The slit control mechanism 857 (also referred to herein as “auto slit”)controls the amount of light that illuminates the zone of excitation ZEincluding the shock wave 28. In some embodiments, the illuminationsystem 805 is placed between the radiation source 855 and the zone ofexcitation ZE. The slit control mechanism 857 of the SWIM 810 controlsthe amount of light which irradiates the plasma plume 23 and the shockwave 28. In some embodiments, the illumination system 805 includes amovable opaque barrier 854, as depicted in FIG. 8B, having several slits854 a, 854 b, 854 c or openings of different sizes and the slit controlmechanism 857 determines which slit the light beam passes through. When,for example, the controller 850 determines, e.g., via a signal receivedfrom the SWDM 820, that the intensity of light detected at the SWDM 820is lower than the acceptable range, the controller 850 commands the slitcontrol mechanism 857 to move the slits such that a wider slit 854 a isprovided in the path of light in the illumination system 805, allowingmore light to irradiate the zone of excitation ZE, and increasing thedetected intensity. On the other hand, if it is determined that theintensity of light detected at the SWDM 820 is higher than theacceptable range, the controller 850 commands the slit control mechanism857 to move the slits such that a narrower slit 854 c is provided in thepath of light in the illumination system 805, thereby reducing thedetected intensity. In some embodiments, another parameter of the SWIM810 adjusted by the controller 850 is the width of the slit in theopaque barrier 854 in the path of the light beam L exiting theillumination system 805.

While the auto tilt 853 and auto slit 857 are depicted in the FIG. 8A asbeing separate from the radiation source 855, in some embodiments, theauto tilt 853 and the auto slit 857 can be integrated with the radiationsource 855 to form a single device of SWIM 810. In such embodiments, thecoupling between the controller 850 and the SWIM 810 can be suitablymodified to provide the same result as disclosed herein. The controller850, thus, sets the intensity of light detected at the SWDM 820 toenable a stable detection of shock waves over a duration of time.

FIG. 9A shows an optical system for illuminating and imaging the shockwaves in the EUV radiation source in accordance with some embodiments ofthe present disclosure. FIG. 9A includes an SWIM 910, consistent withthe SWIM 810 of FIG. 8A that includes a light source 902, e.g., a pointsource, and a lens 904, and generates a light beam LL consistent withthe light beam L of FIG. 8A. FIG. 9A also includes an SWDM 920,consistent with the SWDM 820 of FIG. 8A that includes a first lens 912,an aperture 914, a second lens 916, and a detector 918. The SWDM 920receives a light beam TT, consistent with the light beam T of FIG. 8A,and generates an image at the detector 918. In some embodiments, asshown in FIG. 9A, the light beam LL is a collimating light beam thatenters the zone of excitation ZE and the light beam TT transmits out ofthe zone of excitation ZE. In some embodiments, the zone of excitationZE includes one or more of the elements of plasma plume 23, thepancake-shaped tin 27, and the shock wave 28. In some embodiments, theentering light beam LL interacts with the shock wave 28 at a point 924of the shock wave 28 and generates the light rays 926 and 928 that arepart of the light beam TT that is transmitted out of the zone ofexcitation ZE. In some embodiments, the optical system of FIG. 9A is aSchlieren photography system.

FIG. 9B shows the aperture 914 of the optical system of FIG. 9A forimaging the shock waves in the EUV radiation source in accordance withsome embodiments of the present disclosure. The aperture 914 is shown inmore detail in FIG. 9B and includes two barriers 934 and 932 forlimiting an opening of the aperture 914. In some embodiments, thebarriers 934 and 932 makes the opening asymmetrical such that the lightray 926 passes through the aperture 914 and the light ray 928 is blockedby the aperture 914. In some embodiments, the asymmetrical aperture 914increases the contrast of the show wave imaged on the detector 918. Insome embodiments, the aperture 914 is designed to increase the contrast.In some embodiments, the detector 918 is a camera, e.g., a digitalcamera.

FIG. 10 shows a system 1000 for measuring a strength and direction of ashock wave, measuring a velocity and direction of droplets, andcontrolling an EUV radiation source, in accordance with some embodimentsof the present disclosure. The system 1000 includes a controller 1040that is consistent with the controller 850 of FIG. 8. The system 1000also includes an analyzer 1020. In some embodiments, the analyzer 1020is implemented by one or more of the processor 800 of FIG. 8A, thecontroller 850 of FIG. 8A, and the controller 750 of FIG. 7B. As shownin FIG. 10, the analyzer 1020 is in communication with the SWDM 920 ofFIG. 9A and/or the SWDM 820 of FIG. 8A and receives an image of the zoneof excitation from the detector 918 of FIG. 9A. Also, the analyzer 1020is in communication with the DDM 720 of FIG. 7B and receives an image ofthe path between the droplet generator 115 and the zone of excitation ZEfrom a detector, e.g., a camera, of the DDM 720. In some embodiments,the analyzer 1020 also receives two or more consecutive images of thephotons reflected and/or scattered by the droplets DP in the pathbetween the droplet generator 115 and the zone of excitation ZE.

In some embodiments, the analyzer 1020 determines a shock wave, e.g.,the shock wave 28, in the zone of excitation ZE and also determines aplane of the shock wave 28, a magnitude of the shock wave 28, andvelocity (speed and direction) of expansion of the shock wave 28. Insome embodiments, the analyzer 1020 also determines locations andvelocities of the droplets DP in the in the path between the dropletgenerator 115 and the zone of excitation ZE. In some embodiments, theanalyzer 1020 determines that the shock wave 28 expands in a directionto impact the next droplets DP and alter the direction of the nextdroplets DP. In some embodiments, the analyzer 1020 determines that theimpact of the shock wave 28 and the altering of the direction of thenext droplets DP causes the next laser beam LR2 to miss the next dropletDP that the pulsed laser fires. In some embodiments, the consecutiveimages of the shock wave 28 are captured by SWDM 820 or SWDM 920 and theanalyzer 1020 determines the velocity of expansion of the shock wave 28based on the captured images.

In some embodiments, based on determining the laser beam LR2 will missone of the next droplets DP, the analyzer 1020 sends a droplet-misssignal 1050 to the controller 1040. Upon receiving the droplet-misssignal from the analyzer 1020, the controller 1040 sends one or morecommands to the laser generator 310 and/or the droplet generator 115. Insome embodiments, the controller 1040 commands the droplet generator 115to apply a first delay in the release of the next droplet DP and/or tomodify, e.g., reduce, the speed of the next droplet DP when released. Insome embodiments, the controller 1040 commands the laser generator 310to apply a second delay in firing the next pulse of the laser beams LR2and/or LR1. In some embodiments, when the analyzer 1020 determines thatthe shock wave 28 has cleared the path between the droplet generator 115and the zone of excitation ZE, the analyzer 1020 clears the droplet-misssignal 1050 and in response the controller 1040 commands the lasergenerator 310 and the droplet generator 115 to apply no delay. In someembodiments, the first and the second delay is determined by either ofthe analyzer 1020 or controller 1040 such that the path between thedroplet generator 115 and the zone of excitation ZE is clear of theshock wave 28 when the next droplet DP reaches each point of the pathbetween the droplet generator 115 and the zone of excitation ZE. Asnoted, in response to the command to the droplet generator 115, thedroplet generator 115 may apply the first delay to when the voltage isapplied to the compression controller device 732 of the dropletgenerator 115 to release the next droplet DP. Also, in response to thecommand to the droplet generator 115, the droplet generator 115 maymodify the voltage that is applied to the compression controller device732 to modify the speed of the next droplet DP when released. In someembodiments, the analyzer 1020 and the controller 1040 are combined intoone device such that the controller 1040 includes the analyzer 1020.

FIG. 11 illustrates a flow diagram 1100 of an exemplary process fordetecting a shock wave in a EUV radiation source, in accordance withsome embodiments of the present disclosure. In operation S1110, a targetdroplet, e.g., droplet DP of FIG. 1, is irradiated in an EUV radiationsource 100 (light source) of an extreme ultraviolet lithography toolwith ionizing radiation to generate a plasma. As shown in FIG. 1, alaser beam LR2 irradiates the droplet DP to generate a plasma, e.g., theplasma plume 23 shown in FIG. 2. A shock wave generated by the plasma isdetected, in operation S1120. In some embodiments, the optical system ofFIG. 9A is used for detecting the shock wave. In some embodiments, thedetector 918, e.g., a camera, of FIG. 9A captures an image of the zoneof excitation ZE and the analyzer 1020 of FIG. 10 detects a shock wave28 in the captured image. Next, one or more operating parameters of theEUV light source 100 is adjusted based on the detected shock wave. Insome examples, the detection of the shock wave changes the frequency ofthe generation of the droplets DP such that a shock wave generated by atarget droplet may not affect the trajectory of the next droplet DP.

FIGS. 12A and 12B illustrate an apparatus 1200 for controlling andmonitoring an EUV lithography system, in accordance with someembodiments of the present disclosure. In some embodiments, theapparatus 1200 is used for monitoring shock waves that are generated inthe EUV lithography system and to control the EUV lithography system asdescribed above such that the shock waves do not interfere with thedroplets DP when the droplets DP travel between the droplet generator115 and the zone of excitation ZE. Thus, in some embodiments, theapparatus 1200 performs the functions of the controllers 750 and 850,the synchronizer 730, and the processor 800. FIG. 12A is a schematicview of a computer system that performs the monitoring of droplets andthe shock waves in the EUV lithography system and controlling theparameters of the EUV lithography system that include the generation ofdroplets and the generation of the ionizing laser pulses. All of or apart of the processes, method and/or operations of the foregoingembodiments can be realized using computer hardware and computerprograms executed thereon. The operations includes detecting the shockwaves and determining the parameters of the shock waves. In FIG. 12A, acomputer system 1200 is provided with a computer 1201 including anoptical disk read only memory (e.g., CD-ROM or DVD-ROM) drive 1205 and amagnetic disk drive 1206, a keyboard 1202, a mouse 1203, and a monitor1204.

FIG. 12B is a diagram showing an internal configuration of the computersystem 1200. In FIG. 12B, the computer 1201 is provided with, inaddition to the optical disk drive 1205 and the magnetic disk drive1206, one or more processors, such as a micro processing unit (VIPU), aROM 1212 in which a program such as a boot up program is stored, arandom access memory (RAM) 1213 that is connected to the MPU 1211 and inwhich a command of an application program is temporarily stored and atemporary storage area is provided, a hard disk 1214 in which anapplication program, a system program, and data are stored, and a bus1215 that connects the 1VIPU 1211, the ROM 1212, and the like. Note thatthe computer 1201 may include a network card (not shown) for providing aconnection to a LAN.

The program for causing the computer system 1200 to execute thefunctions of an apparatus for performing the monitoring of the dropletsDP and monitoring shock waves in the foregoing embodiments may be storedin an optical disk 1221 or a magnetic disk 1222, which are inserted intothe optical disk drive 1205 or the magnetic disk drive 1206, andtransmitted to the hard disk 1214. Alternatively, the program may betransmitted via a network (not shown) to the computer 1201 and stored inthe hard disk 1214. At the time of execution, the program is loaded intothe RAM 1213. The program may be loaded from the optical disk 1221 orthe magnetic disk 1222, or directly from a network. The program does notnecessarily have to include, for example, an operating system (OS) or athird party program to cause the computer 1201 to execute the functionsof the photo mask data generating and merging apparatus in the foregoingembodiments. The program may only include a command portion to call anappropriate function (module) in a controlled mode and obtain desiredresults.

In some embodiments and returning back to FIG. 7B or 8A, the DDM 720 orSWDM 820 includes a photodiode designed to detect light having awavelength of the light from the DIM 710/SWIM 810. In some embodiments,the DDM 720/SWDM 820 further includes one or more filters for filteringcertain frequencies of light. For example, in an embodiment, the DDM720/SWDM 820 includes a filter for blocking deep ultraviolet (DUV)radiation. In another embodiment, the DDM 720/SWDM 820 includes a filterfor blocking all frequencies other than that of the light from the DIM710/SWIM 810.

Referring back to FIG. 7B, in some embodiments, the controller 750 is alogic circuit programmed to receive a signal from the DDM 720, anddepending on the received signal transmit control signals to one or morecomponents of the EUV radiation source 100 to automatically adjust oneor more operating parameters of the EUV radiation source 100. Referringback to FIG. 8A, in some embodiments, the controller 850 is a logiccircuit programmed to receive a signal from the SWDM 820, and dependingon the received signal transmit control signals to one or morecomponents of the EUV radiation source 100 to automatically adjust oneor more operating parameters of the EUV radiation source 100. In someembodiments, the controller 750 and the controller 850 are combined intoa same controller. In some embodiments, the controller 750 and thecontroller 850 are combined are combined into processor 800.

As discussed above, the momentum carried by the shock wave 28 may betransferred to the next droplet DP or the pancake-shaped tin 27 andcause a position deviation such that the next laser pulse may notefficiently hit the next droplet DP or the next pancake-shaped tin 27and may even miss the next droplet DP 27 or the next pancake-shaped tin27. Not efficiently hitting and missing the next droplet DP 27 or thenext pancake-shaped tin 27 may generate more debris which may deposit onthe reflective surface of the collector mirror 110, therebycontaminating the reflective surface of the collector mirror 110 andlowering the quality of the reflected EUV light rays. Therefore, bycontrolling the generation of the droplets DP and the laser pulses ofthe laser beam LR2, the effect of the shock wave 28 may be minimized,less debris may be deposited on the surface of the collector mirror 110,and the quality of the reflected EUV light rays are not deteriorated.

According to some embodiments of the present disclosure, a methodincludes irradiating a target droplet in an extreme ultraviolet (EUV)light source apparatus of an EUV lithography tool with ionizingradiation to generate EUV radiation and a plasma. The method alsoincludes detecting a shock wave generated by the plasma and adjustingone or more operating parameters of the EUV light source apparatus basedon the detected shock wave. In an embodiment, the method furtherincludes irradiating the shock wave with a non-ionizing light from ashock wave illumination module and capturing one or more images of theshock wave by a shock wave detection module. In an embodiment, themethod further includes capturing consecutive images of the shock wave,detecting shock wave parameters based on the captured images of theshock wave, and adjusting the one or more operating parameters of theEUV light source apparatus based on the shock wave parameters. In anembodiment, the shock wave parameters include a direction of expansionand a velocity of expansion of the shock wave. In an embodiment, the oneor more operating parameters of the EUV light source apparatus includeone or more parameters of a target droplet generator and a source of theionizing radiation. In an embodiment, the one or more operatingparameters of the EUV light source apparatus include a first time delaybetween ionizing radiation pulses of the source of the ionizingradiation and a second time delay between droplets generated by thetarget droplet generator. In an embodiment, by adjusting one or both ofthe first time delay and the second time delay, the shock waveassociated with the target droplet avoids impacting a next droplet. Inan embodiment, the method further includes irradiating, with anon-ionizing light from a droplet illumination module, a path between adroplet generator and a zone of excitation. The zone of excitation iswhere the target droplet is irradiated by the ionizing radiation. Themethod also includes detecting a light reflected and/or scattered bydroplets in the path to capture one or more images of the path by adroplet detection module. In an embodiment, the method further includescapturing consecutive images of the path by the droplet detectionmodule, determining droplet parameters of at least one droplet based onthe captured images, and adjusting the one or more operating parametersof the EUV light source apparatus based on the droplet parameters. In anembodiment, a source of the ionizing radiation is a pulsed laser. In anembodiment, a source of the non-ionizing light of the dropletillumination module is a first laser and a source of the non-ionizinglight of the shock wave illumination module is a second laser.

According to some embodiments of the present disclosure, a methodincludes irradiating a zone of excitation of an EUV light source. Atarget droplet interacts with an ionizing radiation at the zone ofexcitation and creates a plasma. The method includes determining whethera shock wave is generated by the plasma and adjusting one or moreoperating parameters of the EUV light source based on the determination.In an embodiment, the method further includes irradiating a path betweena droplet generator and the zone of excitation and detecting a lightreflected and/or scattered by droplets in the path to capture one ormore images of the path by a droplet detection module. In an embodiment,the method further includes capturing consecutive images of the shockwave by a shock wave detection module, detecting shock wave parametersbased on the captured images of the shock wave, and adjusting the one ormore operating parameters of the EUV light source based on the shockwave parameters.

According to some embodiments of the present disclosure, an apparatusfor monitoring shock waves in an extreme ultraviolet light sourceincludes a shock wave illumination module including a radiation sourceto illuminate a zone of excitation. A target droplet interacts with anionizing radiation at the zone of excitation to generate an extremeultraviolet light and a plasma. The apparatus further includes a shockwave detection module to capture one or more images of the zone ofexcitation. The apparatus also includes a controller coupled to theshock wave illumination module and the shock wave detection module todetermine whether a shock wave is generated based on the captured imagesof the zone of excitation, determine shock wave parameters of the shockwave, and adjust one or more operating parameters of the extremeultraviolet light source based on the shock wave parameters. In anembodiment, the apparatus further includes a droplet illumination moduleincluding a radiation source to illuminate a path between a dropletgenerator and the zone of excitation and a droplet detection module todetect a light reflected and/or scattered by droplets in the path tocapture one or more images of the path. The controller furtherdetermines droplet parameters of at least one droplet based on thecaptured images of the path and adjusts the one or more operatingparameters of the extreme ultraviolet light source based on the dropletparameters. In an embodiment, the one or more operating parameters ofthe extreme ultraviolet light source includes a first time delay betweenionizing radiation pulses of a source of the ionizing radiation and asecond time delay between droplets generated by a target dropletgenerator. In an embodiment, the radiation source of the shock waveillumination module and the radiation source of the droplet illuminationmodule are non-ionizing. In an embodiment, the radiation source of theshock wave illumination module and the radiation source of the dropletillumination module are non-ionizing lasers, and at least one of thenon-ionizing lasers have a wavelength of about 1064 nm. In anembodiment, the apparatus further includes a synchronizer to synchronizethe droplet illumination module with the droplet detection module andalso to synchronize the shock wave illumination module with the shockwave detection module.

The foregoing outlines features of several embodiments or examples sothat those skilled in the art may better understand the aspects of thepresent disclosure. Those skilled in the art should appreciate that theymay readily use the present disclosure as a basis for designing ormodifying other processes and structures for carrying out the samepurposes and/or achieving the same advantages of the embodiments orexamples introduced herein. Those skilled in the art should also realizethat such equivalent constructions do not depart from the spirit andscope of the present disclosure, and that they may make various changes,substitutions, and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A method, comprising: irradiating a first target droplet in an extreme ultraviolet (EUV) light source apparatus of an EUV lithography tool with a laser beam to generate a plasma and a shock wave generated by the plasma; illuminating the shock wave with illumination light; detecting the shock wave generated by the plasma of the first target droplet using a shock wave detection module; determining an intensity of light detected at the shock wave detection module; and when the intensity of light detected at the shock wave detection module is less than a first threshold amount or greater than a second threshold amount adjusting an amount of illumination light.
 2. The method according to claim 1, wherein the illumination light is non-ionizing light.
 3. The method of claim 1, further comprising: capturing consecutive images of the shock wave; detecting shock wave parameters based on the captured images of the shock wave; and adjusting one or more operating parameters of the EUV light source apparatus based on the shock wave parameters.
 4. The method of claim 3, wherein the one or more operating parameters of the EUV light source apparatus comprise one or more parameters of one or both of a droplet generator or a source of the laser beam.
 5. The method of claim 4, wherein the one or more operating parameters of the EUV light source apparatus comprise one or both of a first time delay between pulses generated by the source of the laser beam or a second time delay between droplets generated by the droplet generator.
 6. The method of claim 5, wherein by adjusting one or both of the first time delay and the second time delay, an impact of the shock wave associated with the first target droplet on a next droplet is avoided.
 7. The method of claim 4, wherein the illumination light illuminates a path between a droplet generator and a zone of excitation, wherein the zone of excitation is where the first target droplet is irradiated by the laser beam; and the method further comprises detecting a light reflected and/or scattered by droplets in the path to capture one or more images of the path by an image sensor of a droplet detection module.
 8. The method of claim 7, further comprising: capturing consecutive images of the path by the droplet detection module; determining droplet parameters of at least one droplet based on the captured images; and adjusting the one or more operating parameters of the EUV light source apparatus based on the droplet parameters.
 9. A method, comprising: irradiating a first target droplet in a zone of excitation of an EUV light source with a laser beam, to create a plasma and a shock wave generated by the plasma; illuminating the shock wave using an illumination light; determining an intensity of the illumination light at the shock wave; and when an intensity of the illumination light is below a threshold amount increasing an amount of the illumination light, and when an intensity of the illumination light is greater than a threshold amount decreasing the amount of the illumination light.
 10. The method of claim 9, further comprising: irradiating a path between a droplet generator and the zone of excitation; and detecting a light reflected and/or scattered by droplets in the path to capture one or more images of the path by an image sensor of a droplet detection module.
 11. The method of claim 9, further comprising: capturing consecutive images of the shock wave by an image sensor of a shock wave detection module; detecting shock wave parameters based on the captured images of the shock wave; and adjusting one or more operating parameters of the EUV light source based on the shock wave parameters.
 12. The method of claim 11, wherein the one or more operating parameters of the EUV light source comprise one or more parameters of one or both of a droplet generator or a source of the laser beam.
 13. The method of claim 12, wherein the one or more operating parameters of the EUV light source comprise one or both of a first time delay between pulses generated by the source of the laser beam or a second time delay between droplets generated by the droplet generator.
 14. The method of claim 13, wherein by adjusting one or both of the first time delay and the second time delay, an impact of the shock wave associated with the first target droplet on the next droplet is avoided.
 15. An apparatus for monitoring shock waves in an extreme ultraviolet light source, comprising: a shock wave illumination module comprising a radiation source configured to illuminate a zone of excitation with an illumination light, wherein a first target droplet interacts with a laser beam at the zone of excitation to generate a plasma; a shock wave detection module comprising one or more image sensors and configured to capture one or more images of the zone of excitation; and a controller coupled to the shock wave illumination module and the shock wave detection module and configured to: determine whether an intensity of the illumination light at the zone of excitation is lower than a first threshold amount or greater than a second threshold amount; and when the intensity of the illumination light at the zone of excitation is lower than a first threshold amount or greater than a second threshold amount, command a slit control mechanism to adjust the intensity of the illumination light at the zone of excitation.
 16. The apparatus of claim 15, further comprising: a droplet detection module configured to detect a light reflected and/or scattered by droplets; wherein the controller is further configured to: determine droplet parameters of at least one droplet based on the light reflected and/or scattered by the droplets; and adjust one or more operating parameters of the extreme ultraviolet light source based on the droplet parameters.
 17. The apparatus of claim 16, wherein the one or more operating parameters of the extreme ultraviolet light source comprise both of a first time delay between pulses generated by a source of the laser beam and a second time delay between droplets generated by a droplet generator.
 18. The apparatus of claim 17, further comprising a droplet illumination module comprising a non-ionizing radiation source configured to illuminate a path between the droplet generator and the zone of excitation.
 19. The apparatus of claim 18, further comprising a synchronizer that is configured to synchronize the droplet illumination module with the droplet detection module.
 20. The apparatus of claim 15, further comprising a synchronizer that is configured to synchronize the shock wave illumination module with the shock wave detection module. 